As interference measuring devices equipped with interference optical systems, those disclosed in Patent Literatures 1 and 2 have been known. Each of the devices disclosed in these literatures splits light outputted from a light source into two, so as to output first and second branched beams, causes a first reflected beam occurring when the first branched beam is reflected by a first subject and a second reflected beam occurring when the second branched beam is reflected by a second subject to interfere with each other, and detects the intensity of thus interfering light with a detecting unit.
The device disclosed in Patent Literature 1, in which one of the first and second subjects is a reflecting mirror while the other is a film-like object to be measured, measures the thickness of the object to be measured. For this purpose, the device roughly adjusts an optical path length L1 from the light source to the detecting unit by way of the first subject with a stepping motor, finely adjusts an optical path L2 from the light source to the detecting unit by way of the second subject with a piezoelectric actuator, determines two positions where the interfering light intensity detected by the detecting unit is peaked, and measures the thickness of the object to be measured from these two positions.
The device disclosed in Patent Literature 2, in which one of the first and second subjects is a reflecting mirror while the other is a film-like object to be measured, measures the surface form of the object to be measured. For this purpose, the device sets the difference (optical path length difference) between optical path lengths L1, L2 to a plurality of target values in sequence, captures respective interference patterns occurring when the optical path length difference is set to the target values with an imaging unit, and measures the surface form of the object to be measured according to thus captured interference patterns. In order for the imaging unit to capture the interference patterns, an imaging optical system which focuses the interfering light outputted from an interference optical system onto an imaging surface of the imaging unit is used.
Patent Literature 1: Japanese Patent Application Laid-Open No. 2005-283387
Patent Literature 2: Japanese Patent Application Laid-Open No. 2001-066122